A coastline of displaced giants
Along the southern shores of Sumba, an island in Indonesia's East Nusa Tenggara province, the landscape holds evidence of immense natural forces. Scattered across the coastal plains, sometimes hundreds of meters from the sea, are massive boulders composed of coral. These are not rocks that rolled down from hills; Sumba is a non-volcanic island and the boulders' composition matches the offshore coral reefs. These are tsunami deposits, blocks of reef ripped from the seabed and carried ashore by ancient waves of incredible size and power.
Geologists have found multiple fields of these enigmatic boulders along Sumba's coast. Some individual blocks are estimated to be 4 by 3 by 1 meters. The presence of these boulders so far inland provides a physical record of prehistoric tsunamis, creating a geological archive of events that occurred long before human record-keeping. They are a record of the energy generated by the nearby Sunda megathrust fault.
Reading the geologic record
The island of Sumba lies in a tectonically active region, situated in the forearc of the Sunda-Banda volcanic arc. To its west, the Indian-Australian plate subducts beneath the Eurasian plate along the Java Trench, a massive fault system known as the Sunda megathrust. This 5,500-kilometer-long fault is one of the most seismogenic structures on Earth, capable of producing earthquakes of magnitude 9.0 or greater. The 2004 Indian Ocean earthquake and tsunami originated from this very fault system.
The Sumba boulders are direct evidence of past megathrust ruptures. Scientists study these deposits to understand the frequency and magnitude of past tsunamis. By using techniques like uranium-thorium dating on the coral skeletons within the boulders, researchers can determine when the coral was alive and constrain the age of the tsunami event that displaced it. Studies of sedimentary layers along the coasts of Sumba and neighboring islands suggest at least two mega-tsunami events may have occurred in the last millennium. Evidence from nearby Sumbawa points to tsunamis in the 4th, 9th, and 17th centuries CE. This long-term geological data surpasses any written historical accounts, providing more data of the region's seismic hazards. This information is important for creating more accurate tsunami hazard maps and improving early warning systems for coastal communities throughout the Indian Ocean.